Shares of embattled commodity trader Noble Group plunged nearly 32 percent on Tuesday, hit by a credit rating downgrade and a report a potential strategic investor may have walked away.

Noble shares plunged as much as 31.6 percent to as low as 40 Singapore cents, before the company requested a trading halt at 9:36 a.m. HK/SIN, pending an announcement.

Representatives of Noble didn't immediately respond to an emailed request for comment.

Earlier, the Singapore stock exchange, SGX, had sent the company a query seeking an explanation for the stock's sudden moves.

The stock has long been beset both by the long downtrend in the commodity markets and by a gadfly anonymous researcher, called Iceberg Research, which has published numerous negative blog posts about the company.

Iceberg has alleged accounting problems at Noble, which the company has repeatedly, strenuously denied.

After the market close on Monday, S&P Global Ratings downgraded Noble's outlook to CCC-plus from B-plus, with a negative outlook, citing weak liquidity and increased refinancing risks after the company unexpectedly reported a loss for the January-to-March quarter.

"We downgraded Noble because we believe the company's capital structure is not sustainable," S&P said in a statement.

"Noble could face liquidity shortfalls in the next 12 months if the Hong Kong-based commodity trading company continues to make losses and it is unable to stabilize its profitability."

S&P noted that Noble has three major maturities over the next 12 months, including $656 million due this year, $379 million in March 2018 and $1.1 billion in May 2018.

The negative sentiment on Noble shares was likely compounded by a Reuters report, citing three sources familiar with the matter, that Chinese state-owned company Sinochem was no longer pursuing an investment in the Singapore-listed company, due to concerns over its finances.

Sinochem turned cautious on Noble after its surprise quarterly loss and warning it wouldn't be profitable for the next two years, the report said.

In February, Noble said in an announcement via SGX that it was involved in discussions on a strategic investment, but it didn't name the potential investor. Media reports citing unnamed sources had said that was Sinochem.

Earlier this month, Noble reported a surprise loss of $129 million for the January-to-March quarter, compared with a $40 million profit in the year-earlier period.